1. Field of the Invention
The present invention relates generally to an image coding system and, more particularly, to an image coding system for carrying out a coding process based on motion in a temporally changing image.
2. Description of the Prior Art
Pixel illuminance data of images is typically transmitted as a sequence of digitally encoded frames. Each frame contains sufficient pixel data for complete coverage of a video display. In the simplest case, a frame contains pixel data for all of the pixels in a display. For television applications, frames are transmitted at a rate of thirty frames per second. Given the large number of pixels in a typical display and given the rate at which the frames are transmitted (i.e., the frame rate), video transmission generally requires the transmittal of large amounts of data. Hence, image data is typically coded to compress the data so that the data may be more efficiently transmitted. The amount of compression realized when coding is performed varies in accordance with a number of factors, including the coding method employed, the amount of movement in the image, the spatial resolution, and the temporal resolution.
The amount of data sent per frame affects the quality of the reproduced image. For example, as shown in FIG. 1, when the amount of data included per frame is increased (see the portion of the curve 2 where the frame rate, expressed as I/B.sub.m, where B.sub.m is a target information amount, is low), spatial resolution increases and, thus, the image quality (i.e., the signal to noise ratio (S/N)) of each frame is raised. However, the number of frames that can be transmitted per unit time is reduced (i.e., the frame rate decreases) so that temporal resolution drops. On the other hand, when the amount of data per frame decreases, the image quality also decreases, but the motion follow-up performance increases as shown by the curve 2 in FIG. 1.
Another approach to reducing the amount of data in a transmitted frame is to use a filter that removes a percentage of the image data. For example, suppose that the data is the illuminance difference between corresponding pixels in a present frame and a preceding frame. The amount of data that is transmitted may be reduced by increasing a threshold value, which the illuminance differences must exceed to be transmitted (curve 6 in FIG. 2 shows an example wherein the filter threshold is raised from T.sub.1 to T.sub.2). As a result, the transmitted information amount decreases from B.sub.1 to B.sub.2. The thresholds are switched to raise the efficiency of the image transmission based on the image to be transmitted.
The switching of thresholds to regulate information amount may be performed by an image transmission controller such as disclosed in Japanese Patent Laid-open No. HEI 3-295392. The system of that patent includes a receiving side where the image is reproduced from the transmitted data. This receiving side operates by determining a productive data amount for a frame. This data amount is then sent to the sending side, wherein coding processing is carried out according to the data amount specified by the receiving side. FIG. 2 shows an example of the threshold values that must be selected to realize this data amount B.sub.m for different images. In particular, for the image represented by curve 8, threshold T.sub.1 must be chosen in order to realize the specified data amount B.sub.M. In contrast, for the image of curve 4, a higher threshold, T.sub.2, must be chosen to realize the data amount B.sub.M.
In this system, the data for switching thresholds must be sent from the receiving side to the sending side. Hence, in a system lacking a data transmission means, switching of thresholds cannot be carried out. Further, in the case where the data amount is changed by only changing the threshold value, as described above, a delicate image quality and motion follow-up performance is difficult to achieve, and, thus, the efficiency of the image data transmission is poor.